This invention relates to the monitoring of gases to detect levels of volatile organic compounds, and, more particularly, to a sensor apparatus useful in such monitoring.
Volatile organic compounds (VOCs) are associated with many industrial processes. VOCs such as solvents (e.g., benzene, toluene, xylene, ketones, acetates, alcohols) are typically used in the processes in liquid form, but may evaporate into the process environment. Some of the VOCs are health hazards and/or contribute to the formation of atmospheric pollutants.
In one example, industrial painting operations utilize large amounts of VOC solvents as carriers for pigments. The solvents evaporate to some degree during the painting as a result of the use of mildly elevated temperatures and/or the use of techniques such as spray painting which utilize finely divided droplets containing the solvents. Great care is taken to confine the solvents to closed systems, but inevitably there is some leakage at joints in the apparatus, input/output ports, and other locations.
Measuring the presence and concentrations of VOCs is of importance in several ways. For example, regulators compare the concentrations of VOCs to regulatory standards to ensure compliance of industrial operations to the standards, and to determine whether certain operations may be allowed to continue. Process engineers use information on VOC production to select processes for implementation. VOC information is used to determine whether a process is operating properly, and to identify sources of inefficient operation. Medical personnel seek to correlate human health with VOC concentrations.
The VOCs are typically present in the range of parts per billion or parts per million of the process gas such as air. Instruments are available to make quantitative measurements of the gas for VOCs at such low levels. Examples of such instruments include flame ionization detectors, Fourier transform infrared spectrometers, and instruments using other wet or dry chemical analysis procedures. Such instruments tend to be relatively expensive and bulky, and require trained personnel to operate them. It is therefore not ordinarily possible to place the measurement instruments at all locations where information might be needed, such as at a plurality of different sampling locations around a paint line. Instead, individual gas samples may be transported, or samples may be piped, from the sampling locations to the central measurement instrument. These approaches are accompanied by concerns that the composition of the sample may change during transport, or that the sampling loses its real-time character. The latter is of particular concern for process engineers who use the sampling data to optimize the operation and performance of the industrial system.
There is a need for an improved practical approach to the sampling and measurement of VOCs in industrial and other environments. The present invention fulfills this need, and further provides related advantages.
The present invention provides an apparatus for measuring volatile organic compounds (VOCs) in a real-time manner with excellent sensitivity. The apparatus is relatively inexpensive, so that units may be placed at various locations around an industrial operation to assess production of VOCs in the industrial operation. The apparatus is suitable for measuring a wide variety of VOCs using the same basic structure, modified only as to the sensor selected for each particular VOC. A single instrument may analyze for a number of VOCs simultaneously.
In accordance with the invention, the apparatus for measuring volatile organic compounds in a gas comprises a sensor chamber, which includes a housing having a gas inlet, a gas outlet, and a gas flow path from the gas inlet to the gas outlet, a gas flow path control structure within the housing to alter the gas flow from the gas inlet to the gas outlet, and a set of sensors within the housing and positioned in the gas flow path. Each sensor has an associated sensor electrical output signal that is dependent upon the presence of a respective volatile organic compound in the gas flow. The apparatus optionally includes a pump that removes the gas from the gas outlet of the housing, and an electronic circuit that measures each of the sensor electrical output signals.
The gas flow path control structure is preferably a set of baffles that spread the gas flow through a volume and to the set of sensors. Desirably, the gas flow path control structure and the set of sensors are cooperatively positioned so that the time required for the gas to flow from the gas inlet to each of the sensors is substantially the same. The apparatus may also be provided with appropriate sensors of its conditions, such as temperature and humidity sensors. The conditions of the apparatus, such as its temperature, may be controlled, as with a heating element to control the temperature in a closed-loop fashion. A microcontroller controls and coordinates operation of the apparatus, including the electronics, pump (where present), sensors, heating elements, and the like.
The sensors preferably include electrically conductive polymers whose electrical properties depend upon the presence of the respective volatile organic compounds in the gas flow contacting the polymer. Various polymers whose electrical properties are sensitive to organic compounds, such as methyl isoamyl ketone, xylene, and butyl acetate, are known.
The present approach provides a compact apparatus for measuring VOCs. The apparatus is relatively compact and light, permitting it to be positioned at locations where measurements are required, or for a number of the apparatus units to be positioned at a number of locations where measurements are required. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.